98 PALAEONTOLOGY OF ILLINOIS. 



those of the posterior teeth, except that the longitudinal ridges are less con- 

 spicuous, sometimes being wholly obsolete; the porosity is coarse, more or less 

 angular and irregular, rarely stellate. 



Figure 2, represents the superior surface of a posterior tooth; figures 3, 3 a, 

 an anterior ? tooth; both natural size. 



Formation and locality: Keokuk limestone,. Warsaw, Illinois. 



Deltodus complanatus, N. and W. 



PI. IX, Fig. 4. 



Teeth broadly triangular in outline, narrower end termina- 

 ting in an acute point, the border extremity having one obtuse 

 angle, the other broadly rounded, slightly arched spirally, but 

 as compared with the associated species flat and thin. A low, 

 obtuse ridge passes from the angle of the broader extremity to 

 the pointed end; from this ridge the surface declines rapidly 

 on one side, very gradually on the other, to the lateral margins ; 

 a faint rudimentary ridge being sometimes visible midway of 

 the broader surface, with the higher one converging to the 

 acute extremity; enameled surface smooth and polished, every 

 where finely punctate. 



Figure 4 represents the superior surface in profile, natural size. 

 Formation and locality: Burlington limestone, Henderson county, Illinois. 



Deltodus undulatus, N. and W. 



PL IX, Fig. 5. 



Posterior tooth strongly arched, triangular in outline, its 

 broader extremity somewhat emarginate, one of the angles 

 broadly rounded, the other sub-acute; a prominent rounded 

 ridge passes from the rounded angle of the broader end to the 

 acute extremity, bordered by a deep sulcus, which, starting 

 from the emargination of the broader end, converges to the 

 same point; enameled surface highly polished and shining, 



